Field of the Invention and Related Art Statement
The present invention relates to an apparatus for driving an objective lens for use in reading and writing information out of and onto an optical record medium.
There has been developed an optical information reading and writing apparatus for reproducing and recording information out of and onto an optical record medium by projecting a focused spot of a light beam upon the record medium. In such an optical information reading and writing apparatus, the information is read out and recorded on the record medium along an information track with the aid of an optical head which includes an objective lens for projecting the light beam spot upon the record medium, a mechanism for supporting the objective lens movably in a tracking direction as well as in a focusing direction, and an apparatus for driving the objective lens in the tracking and focusing directions in accordance with tracking error and focusing error, respectively. It should be noted that the tracking direction is perpendicular both to the optical axis of the objective lens and to the track direction in which the information track extends, and the focusing direction is in parallel with the optical axis of the objective lens. When use is made of an optical record disc, the information track is formed as a spiral track or concentric circular tracks. Then, the tracking direction is a tangential direction of the spiral or circular track. The above mentioned objective lend driving apparatus has been proposed in, for example, Japanese Patent Publications Kokai Sho Nos. 59-221,839, 62-149,044, 62-149,045 and 62-149,047.
FIGS. 1 and 2 show a known objective lens driving apparatus disclosed in the Japanese Patent Publication Kokai Sho No. 59-221,839. An objective lens 1 is supported by a lens holder 2, and a focusing coil 3 is wound around the outer side wall of the holder 2. On opposite sides of the holder which are aligned in the track direction are applied respective pairs of tracking coils 4. The holder 2 is connected to a base 5 by means of four resilient wires movably both in the focusing direction and in the tracking direction. To the base 5 is secured a yoke 7 having upright portions 7a, 7b, and permanent magnets 8 are secured to end plates of the yoke. The holder 2 and the yoke 7 are assembled such that portions of the focusing and tracking coils 3 and 4 are situated in magnetic fields formed between the upright portions 7a, 7b of yoke 7 and permanent magnets 8. By conducting electric currents corresponding to the focusing and tracking errors through the focusing and tracking coils 3 and 4, respectively, the holder 2 and thus the objective lens 1 are moved in the focusing and tracking directions, so that the correctly focused light spot is projected on the information track of the optical record medium.
In the known objective lens driving apparatus described above, the movable portion including the objective lens 1, holder 2 and coils 3 and 4 might be rotated about an axis (Y-axis) which is in parallel with the track direction and is perpendicular both to the focusing direction (FO) and to the tracking direction (Tr). Hereinafter this rotating movement is called the rolling resonance. In order to suppress the rolling resonance, there has been proposed to make a center of tracking force acting upon the tracking coils 4 in the tracking direction coincident with the center of gravity (G) of the movable portion so that any moment about the Y-axis could not be generated.
However, even if the center of tracking force is made coincident with the center of gravity of the movable portion, when the movable portion is moved or shifted in the focusing direction due to the focusing servo control, the equivalent center of tracking force is shifted from the center of gravity so that the rolling resonance is generated, because the magnetic flux density in the magnetic gap in which the tracking coils are moved has such a distribution that the magnetic flux density is decreased toward the upper and lower ends of the magnetic gap.
Now the generation of the rolling resonance will be further explained in detail. FIGS. 3 and 4 are schematic views showing the objective lens 1, holder 2, tracking coils 4 and permanent magnet 8 viewed in the Y-axis. The magnetic gap is situated in front of the permanent magnet 8 and is extended in parallel with the plane of the drawing. G denotes the center of gravity of the movable portion comprising the objective lens 1, holder 2, focusing and tracking coils 3 and 4. When an electric current I is conducted through the tracking coils 4 in directions shown by arrows in FIG. 3, in portions 4b of the tracking coils 4 which portions extend in the focusing direction Fo there are generated forces F in the tracking direction Tr, and at the same time in portions 4a and 4c which extend in the tracking direction Tr there are produced forces f.sub.1 and f.sub.2, respectively in the focusing direction Fo. As illustrated in FIG. 3, when a center of the forces F is coincided with a direction which passes through the center of gravity G and is in parallel with the tracking direction Tr and the forces f.sub.1 and f.sub.2 have the same magnitude, there is not produced any moment about the Y-axis, so that the movable portion is shifted only in the tracking direction Tr without causing the undesired rolling resonance.
However, when the movable portion is shifted in the focusing direction Fo, the force f.sub.1 becomes decreased, but the force f.sub.2 becomes increased, so that f.sub.1 &lt;f.sub.2, because the distribution of the magnetic flux density in the magnetic gap is decreased abruptly toward the upper and lower ends of the permanent magnet 4 as depicted in FIG. 5. Further, the center of the force F is shifted downward by a distance .DELTA.Z as illustrated in FIG. 4 with respect to the center of gravity G. Since the forces f.sub.1 and f.sub.2 have the opposite directions, the two sets of tracking coils 4 are subjected to a moment M.sub.1 which is equal to 2(f.sub.2 -f.sub.1)l, wherein l is a distance between the center of gravity G and the center points of the forces f.sub.1 and f.sub.2 measured in the tracking direction Tr. Moreover, due to the shift .DELTA.Z of the movable portion, there is also produced a moment M.sub.2 amounting to 2F.multidot..DELTA.Z. Since the above mentioned moments M.sub. 1 and M.sub.2 have the same direction, there is produced the very large rolling resonance. That is to say, the movable portion is subjected to the rolling resonance which is caused by a sum of the two moments M.sub.1 and M.sub.2. This influence can be expressed by an equivalent shift .DELTA.Z' of the point of the force F with respect to the center of gravity G, said equivalent shift .DELTA.Z' being calculated as follows: ##EQU1## When the equivalent shift .DELTA.Z' becomes large, there is generated a large rolling resonance. It should be noted that the above explained rolling resonance is equally generated when the movable portion is shifted downward in FIG. 3, but the direction of the rolling resonance is inverted.
In the known objective lens driving apparatus just explained above, it is difficult to suppress the generation of the undesired rolling resonance effectively, so that the phase in the frequency characteristic of the tracking servo control fluctuates and the tracking control could not be carried out correctly. Therefore, the yield of the objective lens driving apparatus becomes reduced. In order to solve the above problem, one may consider to increase the height of the magnetic gap such that the central portion of the distribution curve of magnetic flux density has a flat portion and the tracking coils are moved within this flat central portion. However, this solution might induce another problem that the height of the yoke 7 and permanent magnets 8 has to be increased, and thus the height of the whole apparatus could not be made small.